The present invention relates to gas detectors and analyzers.
Gas detectors are employed for detecting the presence of one or more gases. In one common use, a gas detector is used to detect the presence of a gas in ambient air. One type of gas detector includes a sample chamber into which a sample of the ambient air is admitted. A source of radiant energy such as, for example, infra-red energy, is projected through the sample. A detector, responsive to the radiant energy in a spectral region which the gas is expected to absorb, receives the radiant energy after it has traversed the sample one or more times. The presence of the gas is sensed by the reduced response of the detector caused by the absorption of the radiant energy after having passed through the sample chamber.
A single gas may be identified if its absorption occurs in a spectral region not shared by absorption of other gasses in the sample. Spectral filtering reduces the spectral bandwidth of the radiant energy to that in which the single gas is expected to produce absorption. Thus, any reduction in the received signal is accepted as evidence of the existence of the gas. The magnitude of the absorption is evidence of the concentration of the gas. The presence of more than one gas having overlapping absorption spectra prevents this type of identification.
Another type of gas detector, called a photo-ionization detector, ionizes the gas using radiant energy such as, for example, ultra-violet light. A further type, called a flame-ionization detector, ionizes the gas by contacting it with a hydrogen flame. In both types of ionization detectors, the presence of the gas is detected by an increased electric current in an external circuit These detectors share the same problem of discrimination and confusion as noted in the foregoing paragraph.
One method used in the prior art for identification of gasses takes advantage of the different times required for gasses to pass through a column having an active internal surface such as, for example, an internal surface coated with a sorbant material such as a wax or polyester. A fixed volume of air suspected of containing a gas to be tested, is admitted to a long tubular column. Then, a flow of a clean gas such as, for example, clean hydrogen, is pumped at a fixed rate through the column. Different gas components are retarded by the active surface in the column so that they exit spread out in time, in a manner related to the nature of the coating and the molecular structure of the gas components. Air passes through most columns substantially without retardation. A detector sensing the output of the column produces an output signal varying over time in response to the peaks and valleys of gas components exiting the column.
A knowledge of the elution times characteristic of a coating with respect to particular gasses permits discrimination of a specific gas from among a small group of suspected gasses. Overlapping elution times of different gasses complicates the identification process. In general, the detector output signal contains insufficient information from, which the components of an unknown gas mixture can be identified.
Positive identification of gasses in an unknown mixture is possible in a laboratory environment using, for example, a mass spectrometer. Such devices are neither portable, nor useful in the field, where gas leaks must be detected and analyzed Detectors sensing the outputs of elution columns have found use in the field, but their problems of limited identification capability have not been solved. In addition, conventional analysis devices require such large supplies of compressed gas that either their portability or their operating time is limited. Finally, most gas detection and analysis devices avoid effects of ambient temperature changes by using an electric heater to maintain temperature-sensitive portions of the instrument at a constant temperature. Such an electric heater consumes quantities of electricity that are inconsistent with portable operation.